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Benzene C−H Bond Activation in Carboxylic Acids Catalyzed by O-Donor Iridium(III) Complexes: An Experimental and Density Functional Study
| Content Provider | Semantic Scholar |
|---|---|
| Author | Bischof, Steven M. Ess, Daniel H. Meier, Steven K. Oxgaard, Jonas Nielsen, Robert J. Bhalla, Gaurav Goddard, William A. Periana, Roy A. |
| Copyright Year | 2010 |
| Abstract | The mechanism of benzene C−H bond activation by [Ir(μ-acac-O,O,C^3)(acac-O,O)(OAc)]_2 (4) and [Ir(μ-acac-O,O,C^3)(acac-O,O)(TFA)]_2 (5) complexes (acac = acetylacetonato, OAc = acetate, and TFA = trifluoroacetate) was studied experimentally and theoretically. Hydrogen−deuterium (H/D) exchange between benzene and CD_(3)COOD solvent catalyzed by 4 (ΔH^‡ = 28.3 ± 1.1 kcal/mol, ΔS^‡ = 3.9 ± 3.0 cal K^(−1) mol^(−1)) results in a monotonic increase of all benzene isotopologues, suggesting that once benzene coordinates to the iridium center, there are multiple H/D exchange events prior to benzene dissociation. B3LYP density functional theory (DFT) calculations reveal that this benzene isotopologue pattern is due to a rate-determining step that involves acetate ligand dissociation and benzene coordination, which is then followed by heterolytic C−H bond cleavage to generate an iridium-phenyl intermediate. A synthesized iridium-phenyl intermediate was also shown to be competent for H/D exchange, giving similar rates to the proposed catalytic systems. This mechanism nicely explains why hydroarylation between benzene and alkenes is suppressed in the presence of acetic acid when catalyzed by [Ir(μ-acac-O,O,C^3)(acac-O,O)(acac-C^3)]_2 (3) (Matsumoto et al. J. Am. Chem. Soc. 2000, 122, 7414). Benzene H/D exchange in CF_(3)COOD solvent catalyzed by 5 (ΔH^‡ = 15.3 ± 3.5 kcal/mol, ΔS^‡ = −30.0 ± 5.1 cal K^(−1) mol^(−1)) results in significantly elevated H/D exchange rates and the formation of only a single benzene isotopologue, (C_(6)H_(5)D). DFT calculations show that this is due to a change in the rate-determining step. Now equilibrium between coordinated and uncoordinated benzene precedes a single rate-determining heterolytic C−H bond cleavage step. |
| Starting Page | 742 |
| Ending Page | 756 |
| Page Count | 15 |
| File Format | PDF HTM / HTML |
| DOI | 10.1021/om900036j |
| Volume Number | 29 |
| Alternate Webpage(s) | http://www.wag.caltech.edu/publications/sup/pdf/840.pdf |
| Alternate Webpage(s) | http://authors.library.caltech.edu/17704/3/om900036j_si_002.pdf |
| Alternate Webpage(s) | https://doi.org/10.1021/om900036j |
| Language | English |
| Access Restriction | Open |
| Content Type | Text |
| Resource Type | Article |
